Characterization of the Moloney murine sarcoma virus (MSV) mos gene product has been hindered by expression of extremely low levels of mos protein in transformed cells. In order to analyze the v-mos gene product, we have attempted to develop a eukaryotic expression system in which v-mos protein will be produced at elevated levels. Specifically, plasmid expression vectors have been constructed in which v-mos has been placed under the control of the murine beta-globin promoter in various arrangements with or without additional beta-globin coding sequences. This plasmid DNA containing v-mos and plasmid DNA containing the gene coding for the enzyme, adenine phosphoribosyltransferase (aprt), have been simultaneously introduced into murine erythroleukemia (MEL) cells lacking aprt, using calcium phosphate-mediated DNA coprecipitation or spheroplast fusion. Upon induction of differentiation, it was predicted that the exogenously added beta-globin promoter would be activated to express v-mos protein at elevated levels. However, no v-mos specific mRNA was detected in induced aprt+ cell populations containing unrearranged beta-globin/v-mos sequences. A parallel construct in which the bacterial gene, chloramphenicol acetyltransferase (CAT), was substituted for v-mos in between beta-globin promoter and coding sequences, was introduced into MEL cells. Similarly, CAT activity was not observed before or after induction. In conclusion, the lack of expression of v-mos and CAT in this beta-globin system provides indirect support for the presence of additional, unidentified elements which may affect control of gene activation.